1. Field of the Invention
The present invention relates to line pressure control for an automatic transmission and more specifically to a line pressure arrangement which includes a memory the values of which can be updated in order to compensate for unit to unit variation between sensors used in the system.
2. Description of the Prior Art
U.S. Pat. No. 4,730,521 discloses a line pressure control arrangement which is equipped with a memory in which the appropriate line pressure level is recorded in terms of throttle valve opening angle.
However, this arrangement has suffered from the drawback that the line pressure/throttle opening data is fixed and cannot be adjusted to take into account the inevitable unit to unit variation in output in the various sensors used in the system, particularly the throttle valve position sensing devices. Accordingly, at low throttle openings, as the throttle pressure gain is high, the deviations between throttle position sensors is apt to induce the situation wherein the level of the line pressure is excessively increased.
This sudden increase in line pressure level under low load operation causes overly powerful engagement of the friction elements of the transmission for the amount of torque which needs to be transmitted under the circumstances and renders the suppression of shift and selection shock.